In a nuclear reactor, the reactor core contains nuclear fuel which is typically in the form of fuel rods grouped together in fuel assemblies. Groups of fuel assemblies are arranged into a matrix to form a core capable of controlled fission reaction. Each fuel rod is typically a long member approximately 0.4 inches in diameter and 8 to 15 feet long containing fuel usually in the form of a stack of fuel pellets which are surrounded by tubular cladding. The fuel rods which make up an assembly are grouped together to form a plurality of longitudinally extending members which are supported by two parallel end plates, an upper and a lower tie plate. These plates are usually connected to one another by tie rods, or other structural elements.
Each fuel assembly may also include non-fuel bearing members. Examples include guide tubes to form passageways for control rods which assist in controlling the rate of fission, instrumentation tubes for in-core instrumentation, spacer capture rods, and water rods to modify the neutron moderation in the assembly. The spaces between adjacent fuel rods create flow channels through which coolant and/or moderator can circulate. In light water reactors, the coolant and moderator is water. Lateral bracing and spacing of the fuel rods in the fuel assembly are provided by spacers or spacer grids.
The fuel assembly functions in part to maintain the fuel rods in a fixed position, ideally free of vibration and restrained from bowing or other lateral displacement during normal and other operating conditions. In addition, by maintaining the fuel rods in fixed positions, proper cooling and neutron moderation can be achieved. Devices that assist in maintaining the fuel rods in fixed positions in the fuel assembly and which thereby facilitate proper fuel cooling are spacers.
Spacers or spacer grids which provide lateral bracing are typically designed to allow differential axial expansion of the fuel rods. Springs incorporated in the spacer grids are most frequently used to permit some sliding of the fuel rods with respect to the spacer grids. In some of the designs, the spacer grid is free to move axially a small amount to accommodate minor changes in the axial length of the fuel rods during irradiation. By being positioned at regular intervals, spacers maintain rod-to-rod spacing along the length of the fuel assembly.
As is well known, spacer grids are generally built up from a relatively large number of different intricately shaped strips that are fitted together to form spacer cells and subsequently welded. Each spacer cell typically includes dimples and/or springs to maintain the desired rod-to-rod spacing. Thus, such springs and dimples keep the fuel rods in their proper lateral positions. But, under the influence of radiation, the springs are prone to relax and this can lead to undesirable changes in fuel rod pitch (i.e. rod-to-rod spacing) or it may cause gaps or spaces to develop between fuel rods and the springs and dimples, and increases the likelihood that the rods and/or spacer grids will vibrate. Such gaps, changes in fuel rod pitch, and vibration may lead to fuel rod fretting and failure. Damage to spacer springs or dimples during the manufacture of fuel assemblies or damage during subsequent handling operations increase the likelihood of fuel rod fretting failures. Furthermore, as the fuel is irradiated, the fuel rods undergo a shrinkage or diameter reduction known as "creepdown" which can result in gaps between the fuel rod cladding and the springs or dimples which in turn can cause or contribute to fuel rod fretting.
Fuel assemblies which experienced fuel rod fretting failures due to damaged spacer cells have required various repairs to return the fuel assembly to service. Previous repair methods, including special replacement fuel rod designs or side insertable springs or clips required significant engineering, testing, and licensing work in order to provide an adequate repair of the fuel assembly. These repair devices are usually produced on a case by case basis. Accordingly, their feasibility and functionability must be established for each application.
It would be an advantage over the prior art to utilize a method which will eliminate a substantial amount of engineering and licensing work required by simplifying the above-described prior art repair methods and repair apparatuses.